crap I answered the question asked, not the real question
- fp is a pointer to an int (that is what FILE does - it is equivalent to int) so (FILE *) is a file descriptor pointer the equivalent of (int *) ... does not cast well to a char *
fyi: the standard file descriptors are 0 (stdin), 1 (stdout), 2 (stderr)

Thanks, techno, this way i altered xosview to look for different paths for the temperature line . Will post source if i find them. But in xosview it seems to be clumpsy, should look everywhere in /sys for temp_input[0-9] files .